This invention relates to rotary screening of slurries to remove objectionable impurities, and more particularly to vertical pressure pulp screens to remove debris from pulp slurries and paper making stock.
The production of paper making fibers by any known pulping method is incomplete in that certain portions of the wood are not broken down into individual fibers. Such wood portions, which include debris such as shives, bark, slivers, and chop, as well as foreign particles such as sand, grit, and pieces of metal and plastic must be removed from the paper making fibers before the manufacture of paper can begin. This removal can be accomplished either by centrifugal cleaning to separate high specific gravity particles from good paper making fibers or by screening to separate large surface area particles from good paper making fibers.
In recent years, pressurized rotary screens have become the most common type of screening apparatus in use. These screening devices generally have a pressurized housing with a cylindrical screen plate dividing the housing into an inner chamber and an outer annular chamber and a rotor in the proximity of the screenplate. The slurry to be screened is introduced into the inner chamber and has both a rotary and axial velocity imparted to it by the rotor. The cylindrical plate has slots or holes through which the desirable or accepts fibers pass while the undesirable or rejects particles are retained by the cylindrical screen plate. The accepts and rejects streams are then separately removed from the screening apparatus.
In modern rotary screens, two mechanisms are commonly used to maintain the screens in an open or unplugged condition. The first mechanism uses hydraulic action or pulsation to maintain the screens in an open and clean condition by generating an intermittent reverse flow of liquid through the holes or slots. Many modern screens utilize rotary motion to generate a cyclic reduction of pressure on the inlet side of the screenplate to accomplish this reverse flow. Typically, a rotating element equipped with foils or other cleaning structure is located on the inside of the screenplate and moves in proximity to the surface of the screenplate. The shape of the rotating elements is such that they create a positive pressure ahead of them and a suction in their wake. This hydraulic action or pulsation prevents the holes or slots in the screenplate from plugging.
The second mechanism involves rotation of the entire mass of pulp inside the screenplate cylinder at high velocity using a rotor with blades. Adjacent to the screenplate, a mat of fibers forms due to the dewatering action of the screen. At the tip of the rotor blade the fiber mat is rotating at a high speed while at the screenplate holes, the velocity is essentially zero. This results in large shear forces which align the fibers parallel with the screenplate. Since most undesirable impurities in paper pulp slurries are both longer and stiffer than the desirable fibers, the shear forces set up at the screenplate tend to prevent these impurities from passing through the holes.
A number of rotary screen apparatuses utilize the rotational mechanism to remove impurities. Examples of such apparatuses are Cowan, U.S. Pat. No. 3,081,873; Cowan, U.S. Pat. No. 3,245,555; Hooper, U.S. Pat. No. 3,713,536 and Martin, Canadian Pat. No. 1,007,576. However, the early Cowan patents suffer from the disadvantage that they cannot be operated under pressure. The Hooper patent, by adding dilution water through nozzles located at the tip of the rotor blades, tends to force undesirable particles located at a screenplate opening through that opening and into the accepts chamber when a rotor blade passes over the opening. Moreover, none of the above patents makes full use of the presence of the mat of fibers which acts to screen out undesirable fibers.
Accordingly, the need still exists in this art for a rotary pressure screen which can efficiently and effectively achieve essentially complete separation of desirable fibers from undesirable fibers and other particles in a pulp slurry.